High purity polysilicon for the electronic and photovoltaic industries is manufactured by the Siemens process. (Handbook of Semiconductor Silicon Technology, W. C. O'Mara, R. B. Herring and L. P. Hunt, Noyes Publications, 1990.) This technology is basically a hot wire (or rod) chemical vapor deposition (CVD) process, in which silicon is deposited on high purity silicon rods at high temperatures by the following processes:
Decomposition of trichlorosilane (TCS), SiHCl3, at ˜1050-1150 C2SiHCl3→Si+2HCl+SiCl4  (1)
Decomposition of silane, SiH4, at ˜800-900 CSiH4→Si+2H2  (2)
The TCS-based Siemens process is the more prevalent in the industry, wherein electrically heated silicon slim rods at ˜1150° C. are exposed to trichlorosilane (mixed with hydrogen). (Handbook of Semiconductor Silicon Technology, W. C. O'Mara, R. B. Herring and L. P. Hunt, Noyes Publications, 1990.) The trichlorosilane gas decomposes and deposits silicon onto the heated silicon rods, enlarging them according to the chemical reaction (1).
Pure silicon is intrinsically a nonconductor for electricity at room temperature, with resistivity of the order of 20,000 ohm-cm. Thus, direct initial electrical heating of the slim rod to raise its temperature is not practical. Typically external (or internal) radiant heat from a bank of infrared halogen lamps is utilized to preheat and raise the slim rod temperature to ˜400 C. At that temperature, the resistivity of pure silicon decreases to ˜0.1 ohm-cm, and the silicon becomes electrically conductive to facilitate direct electrical heating. Even then a high voltage power supply is required to further increase the temperature of the slim rod. When the silicon rods attain a temperature of ˜800 C the slim rods will have reached the avalanche breakdown temperature. The high electrical conductivity of the silicon at this temperature requires and permits the high voltage power system to be replaced by a lower voltage-high current power source.
Typically, the external radiant heating is terminated soon after the high to low voltage power switching. The process gas feed (TCS+H2 or SiH4+H2) to the reactor is also initiated at this time. Gases are preheated to ˜400 C prior to feeding the reactor. The low voltage-high current electrical power continues to pass directly through the slim rods, and which will enable the slim rods to be heated to the CVD process operating temperature of 1150 C for TCS decomposition or 900 C for silane decomposition. This power supply is also utilized to control the temperature of the deposition process as the slim rod diameters grow in size. Typically high currents, up to 2 kA will flow through a single inverted U-rod. A pyrometer is used to measure and control the temperature of the polysilicon rod during the deposition process. Si grows on the intrinsic Si starting rods, and their diameter increases to ˜150-200 mm.
Thus, the Siemens reactor requires many power supplies: (1) a primary independent power supply for preheating the slim rods to 400 C; (2) a high voltage-low current power supply for the second stage heating of the slim rods to 800 C; and (3) a low voltage-high current power supply for the third stage heating of the slim rods to 1150 C and for the deposition process. The second and third stage power supplies have to be incorporated with high voltage switching gear to permit smooth transfer of the electrical power. Since the current drawn by the silicon slim rods at around 800 C is of a run away nature, the switching of the high voltage to low voltage power source needs to be done with extreme care and caution.
The Siemens process is highly energy consuming. A major part of the energy input to the growth rods (greater than 90%) is dispersed and lost as radiant and convective heat. To avoid deposition of the silicon on the inner walls and surfaces of the deposition chamber the reactor walls are generally cooled to less than 50 C. Most of the energy from the hot silicon rods is radiated to the water-cooled bell jars covering the Siemens reactor.
There is a need to simplify the complex method of heating silicon slim rods in the Siemens reactor system.
A further expensive and energy intensive problem related to trichlorosilane decomposition in a Siemens reactor is the generation of large amounts of unwanted by-products. Other reactions that do not produce silicon and that take place in the reactor are:2SiHCl3→SiH2Cl2+SiCl4  (3)HCl+SiHCl3→SiCl4+H2  (4)
Specifically, several moles of silicon tetrachloride, SiCl4 (STC) are formed for each mole of silicon. The industry typically uses a STC converter, a high temperature hydrogen reactor that consumes considerable energy and only partially converts the STC to TCS, which then needs to be decomposed into polysilicon in another pass through the reactor.